1. Field of Invention
The present invention relates to a charge control circuit; particularly, it relates to such charge control circuit which detects an operation of a transistor on the charging path to generate a non-predetermined voltage for optimum control of the charging operation.
2. Description of Related Art
Rechargeable batteries are commonly used to power portable electronic devices. A battery charger for charging such a rechargeable battery is usually required to power the system load and charge the battery at the same time. During such operation to concurrently power the system load and charge the battery, it is a basic requirement that the current should not flow back from the battery to the system load; otherwise the battery can never be fully charged. Therefore, in the design of the conventional battery charger, a predetermined voltage difference is set between the battery charging voltage and the system input voltage. That is, the battery charging voltage is set to be lower than the system input voltage, such that the current will not flow from the battery to the system load during charging operation.
FIG. 1 shows the relationship between the battery charging voltage Vbat and the system input voltage Vsys, as an external power source provides power to the battery charger. As shown in the figure, there is a voltage difference Vos between the battery charging voltage Vbat and the system input voltage Vsys. The voltage difference Vos is a predetermined value set by a designer, which is usually set conservatively to ensure a safe voltage difference between the battery charging voltage Vbat and the system input voltage Vsys. However, in practical operation, it suffices as long as the battery charging voltage Vbat is smaller than the system input voltage Vsys and there is sufficient current in the charging path. Therefore, by setting the voltage difference Vos to a predetermined level, usually the voltage difference is not optimum and if the predetermined voltage difference is larger than required, there will be unnecessary waste of power; if the predetermined voltage difference is too small, there will be insufficient current in the charging path.
Please refer to FIG. 2 for an example of the above-mentioned prior art, which shows a simplified circuit diagram of U.S. Pat. No. 7,710,079. The charger circuit 10 includes a regulator circuit 11, a charge control unit 12, an internal voltage source 14, a transistor Q1, an error amplifier circuit 13, a system load (having a system input voltage Vsys) and a battery charging terminal Vbat. The regulator circuit 11 is coupled to an external power source Vbus and supplies power to a common node N11. The transistor Q1 is coupled between the common nodes N11 and N12. The charge control unit 12 is coupled to the gate G1 of the transistor Q1. The internal voltage source 14 is coupled between the common node N12 and the error amplifier circuit 13 and it generates an voltage difference Vos. The error amplifier circuit 13 includes one negative input terminal and two positive input terminals, wherein the negative input terminal is coupled to the common node N11, and the two positive input terminals are coupled to the internal voltage source 14 and a reference voltage Vref, respectively, wherein the reference voltage Vref corresponds to a voltage level of 3.4V shown in FIG. 1. When the charger circuit 10 is in the charging stage I, the error amplifier circuit 13 compares the voltage level Vsys of the system load with the reference voltage Vref to generate a control signal which controls the voltage supplied by the regulator circuit 11 according to the reference voltage Vref. When the charger circuit 10 is in the charging stage II, the error amplifier circuit 13 compares the voltage level of the system load Vsys with the internal voltage source 14 (the output voltage of the internal voltage source 14 is the sum of the battery charging voltage Vbat and the voltage difference Vos) to generate a control signal which keeps a predetermined voltage difference between the voltage supplied by the regulator circuit 11 and the battery charging voltage Vbat in correspondence with the voltage difference Vos. However, the predetermined voltage difference is not always an optimum voltage as desired. If the predetermined voltage difference is set too large, there will be unnecessary power consumption. If the predetermined voltage difference is set too small, the transistor Q1 will not be completely turned ON.
Therefore, it is desired to prevent the current of the battery from flowing back to the system load under the charging condition, while improving the charging efficiency by reducing the power consumption due to the above-mentioned voltage difference.